Superconducting magnet systems typically comprise coils of superconductive material wound on suitable formers mounted in a sealed cryogenic environment. These systems generate extremely intense magnetic fields and require precise and careful control, particularly when it is necessary to deliberately extinguish the magnetic field in order (for example) to attend to planned maintenance, or to effect repairs, or to meet changing operational requirements. For such planned operations, the magnetic field is usually extinguished by a controlled ramping down: that is, the electrical current flowing in the magnet coils is gradually reduced to or towards zero by extraction of current through current leads electrically connected to the magnet. This takes some time and requires effective control to be in place.
However, an emergency situation may arise in which it is necessary to quickly extinguish the magnetic field and/or to extinguish the magnetic field when the control systems are inoperational. For example, such situations may include: in order to release magnetic parts which have been unintentionally attracted to the magnet, possibly trapping a person close to the system; or in order to avoid the risk of metallic parts being attracted—for example when fire-fighters have to approach an active magnet with their full equipment, which contains bulky magnetic components. For such emergency operation, it is essential that the run-down unit should be capable of effectively initiating a quench in the magnet without mains electrical power. In situations such as a fire being present, the mains electrical power is likely to be lost. In the case of a person being trapped by the magnet, a reflex reaction may be to turn off mains electrical power in the mistaken belief that such action would turn off the magnetic field. It is thus necessary to provide reliable, non-mains dependent quench initiation devices as emergency run down units as part of, or in addition to, a controlled quenching system. It is usual for such emergency run down units to initiate a localized quenching event, and then to promote a controlled distribution of quenching events throughout the system.
It is also the case that such superconducting magnets can self-quench in response to a sudden and irreversible collapse of the magnetic field arising from any event which can cause localized disturbance of the superconductive state. This will lead to the creation of electrical resistance at the site of the disturbance, and a rapid build-up of localized heating which exacerbates the resistance. It is well known that, in order to reduce the risk of localized, possibly permanent, damage to the magnet coils, the quench should be distributed as widely as possible through the coils of the system shortly after the commencement of a quenching event. This is typically achieved through use of quench propagation devices, typically electric heaters, as part of a controlled quenching system.
Both eventualities are typically addressed by providing small electrical heaters as quench initiation or propagation devices, located adjacent the magnet coils. The heaters are arranged to be energized, thereby heating the magnet coils to bring them out of their superconducting state and so to raise the resistance of the magnet coils. The energizing may be effected automatically in response to one or more actuation criteria indicating the onset of accidental quenching, or manually in the case of an emergency situation where quenching is required despite the magnet operating correctly.
The amount of power required to initiate a quench is quite small: in the order of two watts for one second, being an energy input of 2J. It is straightforward to provide this from the mains power supply, but as discussed above it is a safety requirement that a quench may be initiated by the emergency run down unit of the present invention even during failure of the mains supply. In known arrangements, a battery back-up is typically provided, for use in energizing quench heaters in the event of a mains failure. However, difficulties and significant expense arise in providing and maintaining a secure and reliable battery powered back-up and, moreover, environmental considerations militate against the use of batteries.
The present invention aims to address one or more of the above issues.
The invention specifically eschews primary reliance on the use of batteries, in which the electrical energy is stored, preferring rather to generate the required power at a time of demand. The invention thus avoids not only the necessity of a back-up battery, but also the expense of providing a reliable maintenance program therefore, and the problems associated with responsible disposal thereof. In particular, the invention provides for the generation of electrical energy when required to cause quenching of the magnet coil, using stored mechanical energy or energy applied by muscular action by an operator.